HTLV-I transformation of human T-cell occurs through the selection of permissive cells, both in vitro and in vivo. We are investigating the viral and cellular genetic determinants involved in the mechanism of human T-cell transformation in vitro, with emphasis on their relevance in the development of adult T-cell leukemia (ATL). Because HTLV-II also transforms T-cells, but does not cause disease, different genetic determinants may be involved in the ability of both viruses to induce T-cell transformation. Indeed, we have found that in HTLV-I a protein encoded from the pX region binds to the IL-2R beta and alpha chains, but none of the proteins from the HTLV-II pX region do. In addition, while in vitro transformation by HTLV-I corresponds to a constitutive activation of the IL-2R (JAK/STAT) pathway, this is not the case in HTLV-II. Interestingly, by studying the functional status of the JAK STAT pathway in uncultured ATL samples, we found that, in some cases, STAT-5 is activated, but in other cases it is not, indicating that biochemical changes in ATL cells differ. To define the viral genes necessary to transform T cells, we have generated a series of mutant HTLV-I proviruses locking one or more genes from the pX region. The ability of the mutant viruses to activate the JAK/STAT pathway is being investigated. Efforts in understanding the role of other viral proteins, as the p13II, have revealed a similarity of this protein with the cytosolic aconitase. Because both p13II and aconitase are also mitochondrial proteins and the latter one is also an RNA binding protein, we are in the process of investigating whether the p13II may bind cellular or viral RNA. To identify which cellular genes may be involved in viral transformation, we studied the status of proteins involved in cell cycle progression in HTLV-I transformed T-cells, and found that p53 is not functional, although it still binds to p53 responsive DNA element and that p21, p16, and p27 are over expressed and associated with CDK2 (p21 and P27), CDK4, and CDK6 (p21, p27, and p16). Because transformed cells still proliferate, we are now to carry on studying the kinase activity associated with CDKs. Finally, efforts in clarifying whether the sites of integration of HTLV-I in the human genome are relevant to the selection of T cells permissive for T-cell transformation are also in progress.